HP 3455A - Page 167

Model
3455A
SERVICE
GROUP G Section VIII
nearest -hp- Service Office, if inoperative. The only checks that can be made are the following.
1. If the reading on the
display
jumps 10 or 100 counts
when
adjusting any adjustments on
the reference assembly, the
wiper
of the pot may be dirty.
Work
the adjustment
screw
of pot
back and forth to clean the wiper.
2. Make sure an oxidation layer has not formed on any pins of the assembly printed circuit
board. The pins can be cleaned
with
a soft lead
eraser.
3. Typically,
the
maximum noise
allowed
on the reference voltages (use a DVM with
input Z
> 1010)
are
20#tV
for the + 10
V
reference and
30
/tV
for the
-10
V
reference. Replace
the assembly if
excessive
noise is
present.
4.
The + 10
V
reference
voltage
at A10TP8 should be adjustable to -f 10 V ±
100 fiV
and the
-10
V
to
-10
V
+20
mV.
Replace the
assembly if
both the
+ 10 V
and
-10
V are not adjustable. Replace A10U7 if only the
-10
V reference
is incorrect.
8-G-4. Turn-Over Errors (Schematic
1,
S, and B).
8-G-S.
Turn-over
errors are present
when,
for example, a
positive
input reading is good and the
negative input reading
is out of
tolerance. This can be checked by taking a positive reading and then
reversing the input leads.
The
following are
a
few turn-over
checks
and
hints.
a. When checking for turn-over errors, the 10
V
range and zero offsets should be the first things to
check.
b. Check the A/D
converter
(A14)
if turn-over differences
arc
observed.
Replace,
if necessary.
c.
Turn-over errors on all ranges:
Unsolder CRl and CR2 from the
AlO
board.
If
the error disappears, CRl
and/or CR2 may be leaky. Make sure the
+ 10 V and
-10
V
references are good
(A10TP8 should be + 10
V
±
100
/»V
and
A10TP7 should be
-10
V
±
20 mV).
d.
Turn-over differences on the
10 V range: Check AlOUl, U2,
Q7,
or QI8.
Other
possible causes may be
Kl, Al,
Q2, Q4,
Q19,
and their associated circuits.
e.
Turn-over errors on the top three ranges: Check A10K6 and QIS.
f.
Turn-over
differences on the l(X)
V
and KXX)
V
ranges:
Lift A10Q9 and
CR29. If the error
disappears, CR31 and/or CR29 may be leaky. Do the
same with
CR16 and CR17.
Q8
may also be
defective.
g.
The FETs connected to A10TP2 may cause turn-over errors, if leaky. (^40 may also be leaky.
h. If the
negative
readings are good and all
positive
readings
above
20
V
are unstable
on
the 100
V
range, check AI0Q36.
B-G-6. Othar Troubleshooting (Schematic B and 111.
a. If the instrument fails to sample in the dc volt, high resolution mode and the ac volt normal
resolution mode, check A1C29 or U48.
b. If the
HP-IB operation
is intermittent
with
the
instrument’s LED’s
dim,
make
sure the
50/60
Hz
switch
is in
the correct position.
c. If the fan refuses to spin after repeated turn-ons, change AIRIS,
19,
and 24 from 11.8 kll to
13.3 kll (0757-0289). This change should not be made on operating fan circuits.
d.
A
good
fan measures approximately 30 (1 between the brown and yellow wires of the fan. A
defective fan will usually measure between 10 (1 and IS Q.
8-77